Block mutates a variable that is indirectly in it's scope - objective-c-blocks

Question

This is the most minimal example I could think of:
#import <Foundation/Foundation.h>
#interface ObjCClass:NSObject
#property void (^aBlockInner) ();
-(void)method;
-initWithBlock:(void (^)())aBlock;
#end
#implementation ObjCClass
- (void)method{
self->_aBlockInner();
}
-(instancetype)initWithBlock:(void (^)())aBlock
{
self->_aBlockInner = aBlock;
return self;
}
#end
struct cppClass{
cppClass(){
objCClassInstance = [[ObjCClass alloc] initWithBlock:^{
modifyY();
}];
[objCClassInstance method];
}
void modifyY() {
y++;
}
int y = 0;
ObjCClass* objCClassInstance;
};
int main()
{
cppClass a{};
NSLog(#"Y is:%d",a.y);
}
The member variable y is supposed to stay untouched as blocks are supposed to copy their “captures”. Though, the final print outputs:
Y is:1
Have I misunderstood Objective-C blocks?
To compile on a macOS do: clang++ main.mm -framework Foundation

Answer 1

Much more minimal example would be as follows:
struct MClass {
int i{};
MClass() {
void(^block)() = ^{
++i;
};
block();
}
};
int main() {
MClass var;
NSLog(#"%d", var.i);
}
This snippet has exactly the same "problem", and i member variable gets changed within a block. This happens, because when you refer to member variables from inside member functions, this implicitly adds this pointer to the expression, so it's not actually i member variable that gets captured, but this pointer which encompasses it. The block above can be equivalently rewritten like this:
void(^block)() = ^{
++this->i;
};
The block makes a copy of the pointer to the same object, not object itself. However, if you had the owning object referred to by value, and not a pointer, you would not be able in fact, alter the object, because the copies a block makes are constant:
struct MClass {
int i{};
};
int main() {
MClass var;
void(^block)() = ^{
// Compile-time error: Read-only variable is not assignable
++var.i;
};
block();
NSLog(#"%d", var.i);
}
And this can only be done with use of the __block modifier:
__block MClass var;
void(^block)() = ^{
++var.i;
};